1. Field of the Invention
This invention relates to the preparation of surfactant-coated, single-walled and multi-walled carbon nanotubes. More particularly, it relates to the preparation of single-walled and multi-walled carbon nanotubes that are dispersible in aqueous and organic solvents to form stable and uniform dispersions.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98.
Carbon nanotubes (CNTs) were discovered in 1991 by S. Iijima (S. Iijima, Nature 354 (1991) 56). They are nanometer-size cylinders comprised of carbon atoms. CNTs can be classified into single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). A single-walled carbon nanotube can be thought of as a sheet of graphite (a hexagonal lattice of carbon atoms) rolled into a cylinder. Typical diameters of SWCNTs are in the range of 0.7-1.4 nm and their length can range from a few tens of nanometers to several micrometers (S. Huang et al., J. Am. Chem. Soc. 125 (2003) 5636), making them one of the highest aspect-ratio objects known. Multi-walled carbon nanotubes can be regarded as a coaxial assembly of SWCNTs. The separation between adjacent tubes is close to the separation found between layers in graphite. Typical diameters of MWCNTs are in the range of 5-50 nm.
Depending on their diameter, length, and chirality, CNTs may exhibit unique optical, electrical, thermal, and mechanical properties. CNTs can behave as semiconductors or as metals (C. Dekker, Physics Today 52 (1999) 22) and their Young's modulus is ˜1.2 TPa, 5 times that of steel, which makes them one of the strongest known objects in nature. CNTs have high thermal conductivity, over 3000 W/m K at room temperature (P. Kim et al., Phys. Rev. Lett. 8721 (2001) 215502). The breaking strength of SWCNTs and MWCNTs can reach 52 and 63 GPa respectively, approximately 30 times that of high-strength steel (M. F. Yu et al., Phys. Rev. Lett. 84 (2000) 5552; M. F. Yu et al., Science 287 (2000) 637). The high aspect ratio (length/diameter) and unique properties of CNTs make them highly desirable for composite materials with significantly improved electrical conductivity, thermal conductivity, mechanical strength, and photonic properties.
However, the advantageous properties of CNTs are often unrealized in composite materials on a macroscopic level for several reasons. The first reason is a tendency of CNTs to crystallize in rope-like structures which become entangled into networks. Strong Van-der-Waals interactions between two CNTs lead to their alignment and to their consequent packing into ropes which may contain 100 to 500 tubes (J. Liu et al., Science 280 (1998) 1253). This aggregation of CNTs affects (often adversely) both their electrical and mechanical behavior (S. Sanvito et al., Phys. Rev. Lett. 84 (2000) 1974). The second reason is the insolubility or poor dispersion of CNTs in common organic solvents and polymer matrixes. The solubility of SWCNTs in common organic solvents is often less than 0.1 mg/ml (J. L. Bahr et al., Chem. Commun. 2 (2001) 193). In many cases, CNTs are heterogeneously dispersed in matrix materials (e.g., polymers), leading to physical (instead of chemical) interactions between the CNTs and the matrix materials.
In order to address these problems, researchers have developed many methods for preparing uniform and stable CNT dispersions. Surface functionalization and the addition of surfactants are perhaps the most frequently used methods. Surface functionalization introduces chemical functional groups onto the surface of CNTs whereas surfactants are usually added to solvents in which CNTs are to be dispersed. Different chemicals and methods have been used for the surface functionalization of CNTs (see, e.g., U.S. Pat. Nos. 6,368,569 and 6,531,513 to Haddon et al.; U.S. Pat. Nos. 6,827,918 and 6,875,412 Margrave et al.; U.S. Pat. No. 7, 247,670 to Malenfant et al.; U.S. Pat. No. 7,250,569 to Sun et al.; U.S. Pat. No. 7,411,085 to Hirakata et al.; U.S. Pat. No. 7,414,088 to Ford et al.; U.S. Pat. No. 7,459,137 to Tour et al.; U.S. Pat. No. 7, 531,157 to Ford et al.; J. Zhang et al., J. Phys. Chem. B 107 (2003) 3712; and, S. Banerjee et al., J. Phys. Chem. B 106 (2002) 12144). The concentrations of these surface-functionalized CNTs in solvents (usually organic) are either low (typically less than 5 mg/ml) or unspecified. Adding surfactants in solvents may also improve the dispersibility of CNTs (see, e.g., U.S. Pat. No. 6,783,746 to Z. Zhang et al.; U.S. Pat. No. 6,878,361 to Clarke et al.; U.S. Pat. No. 7,365,100 to Kuper et al.; U.S. Pat. No. 7,588,941 to Zheng et al.; Q. Xiao et al., J. Inorg. Mater., 22 (2007) 1122; J. R. Yu et al., Carbon 45 (2007) 618; J. I. Paredes et al., Langmuir 20 (2004) 5149). However, CNTs are usually dispersed in aqueous solvents.
In particular, as-prepared liquid dispersions are only stable for a limited time. For example, polyvinylpyrrolidone (PVP)-stabilized SWCNT/N-Methyl-2-pyrrolidone (NMP) dispersions are only stable for about three weeks (T. Hasan et al., J. Phys. Chem. C 111 (2007) 12594). Although the stable time of SWCNT/NMP dispersions can be increased to approximately four weeks, the concentration of SWCNTs in NMP solvents must be less than about 0.05 wt % (Y. Sakakibara et al., U. S. Patent Pub. No. 2007/0224106 A1). For many applications, a dispersion with a high concentration of carbon nanotubes may be required since this decreases the cost, facilitates processing, and lowers the usage of solvents that may not be environmentally friendly.
Thus, it is desirable to make dry, dispersible CNT powders. Dry CNT powders can be re-dispersed in solvents to form CNT dispersions for immediate use. In addition, it is easy to store, transport, and manipulate the dry CNT powders. Qiu et al. (J. Qiu et al., J. Nanopart. Res. 10 (2008) 659) have made dry N-vinylpyrrilidone (NVP)-coated MWCNT powders that are both hydrophilic and lipophilic. The content of these MWNTs in water, alcohol, and dimethylformamide (DMF) is only 0.40, 0.33, and 0.34 mg/ml, respectively, or, when expressed as a weight percentage, 0.040%, 0.042%, and 0.036%, respectively. U.S. Pat. No. 7,501,108 to Yerushalmi-Rozen et al. describes the use of gum arabic (GA) as a surfactant to coat SWNCTs. The as-coated SWCNTs, however, can only be re-dispersed in water, rather than in organic solvents, to form stable dispersions.
None of the conventional methods provides a process for making dispersible CNTs as described and claimed in the instant invention as follows.